1. Field of the Invention
The invention relates to nitro-carburised metal rings for use as piston rings or sealing rings.
Piston rings and sealing rings are commonly made of steel or cast iron and are generally rectangular in cross-section. The ring is located in and projects from a groove and has a radially outer surface in sliding contact with a co-operating surface of, for example, a cast iron cylinder. Two generally radially extending surfaces (herein after called "sides") engage with walls of the groove during the sliding movement. As a result of this both the radially outer surface and the sides are subjected to wear. Various techniques have been proposed for reducing some of this wear in order to increase the life of the ring and particular attention has been given to the reduction of the wear of the radially outer surfaces and the co-operating cylinder or liner. More recently, however, engine life requirements not only reduced wear of the radially outer surfaces but also reduced wear of the sides and the co-operating groove walls.
2. Review of the Prior Art
One technique for reducing wear of the radially outer piston ring surfaces is immersing the rings in a nitro-carburising salt bath containing sodium and potassium salts with the rings heated to a temperature of, say, 400.degree. C. In this nitro-carburising process, certain steels and cast irons of all types, e.g. grey irons, carbidic, martensitic, bainitic and spheroidal (nodular graphitic irons), have nitrogen and carbon simultaneously diffused into their surface to form a hardened surface layer.
British Patent Specification No. 1,576,143 discloses a process of salt bath nitro-carburising the surface of a sintered metal piston ring or sealing ring. The rings are immersed in the salt bath in a stack i.e. with their sides in contact under the pressure of a weight. This is necessary because, if spaced apart, the rings will warp and lose their shape and flatness and also because individual treatment of each ring would be time consuming and expensive.
In this process, however, only the radially outer surfaces of the rings are nitro-carburised, because the rings are in a closed stack. In addition, the use of a salt bath is both slow and messy.
An alternative technique has been chromium plating in which the rings are again placed in a closed stack with their radially extending side surfaces in contact and then plated on their radially outer surfaces with chromium in a conventional way. In order to prevent the plating bridging adjacent rings, it is necessary to chamfer the edges of the rings between the radially outer surface and the sides. This is shown in FIG. 1 which is a photo-micrograph of a part of a cross-section of a piston ring at a corner between a radially outer surface of the ring and a side of the ring.
In this process only the radially outer surfaces of the rings are plated as will be seen from FIG. 1. The sides can be chromium plated in a subsequent plating operation but this is relatively expensive. The chamfered edges of the rings, when in use, tend to increase oil seepage past the rings and thus tend to increase oil consumption, as well as reducing the effectiveness of the seal between the ring and the cylinder so increasing blow-by. Thus chamfers are undesirable. Further, chromium plating softens progressively at temperatures above 250.degree. C. to 300.degree. C. and this is also a disadvantage. In addition, the chromium plated rings require finishing operations which involve lapping and this increases the cost of their production.